Image guided surgery or computer assisted surgery typically utilizes a pre-operative or intraoperative scan or image of a patient to position a device, surgical instrument, or tool during surgery. These images may take various forms and may be acquired using various imaging modalities such as, for example, computerized tomography (CT), magnetic resonance (MR), positron emission tomography (PET), fluoroscopy, x-ray, or other imaging modality. These images are usually taken and transferred to a computer system. The surgical patient may then be moved to an operating room, and a tracking device such as, for example, those using a camera, magnetic field generator, or other device may be connected to the computer system with the images loaded onto it. The tracking device may then track position indicating elements attached to the patient's anatomy to determine the location and orientation of the anatomy. In addition, any tools equipped with position indicating elements may also be tracked and their location and orientation displayed superimposed on a computer display of the scan/image(s).
To prepare for image guided surgery, registration may be performed after pre-operative scanning/imaging. Registration may be performed using various methods depending on the surgical environment, surgical application, or other factors.
There are different ways of registering the position of an anatomical object in a patient coordinate system to views of the anatomical object in an image coordinate system, wherein the anatomical object is relevant to an image guided surgical procedure. The patient coordinate system may be referred to as the “patient space,” and may be measured by a tracking device. The image coordinate system may be referred to herein as the “image space,” and may be provided by an imaging modality.
A first registration procedure may include “point registration.” With point registration, a physician selects landmarks or particular points identified in the pre-acquired images and touches corresponding landmarks or points in the patient's anatomy with a pointer probe having a position indicating element that is tracked by a tracking device. By selecting internal or external anatomical “landmarks” that are identifiable in the pre-acquired images, it is possible to establish a relationship between the two coordinate systems (e.g., the image space of the pre-acquired image, and the patient space of the tracking system). This enables accurate navigation. Often, skin marks are used for this purpose, applied as discrete markers such as ball bearings stuck to the skin.
Another type of registration is a “surface registration” technique. This technique is similar to point registration, except that a tracked pointer probe is dragged or passed along the patient's anatomy, either internally or externally. By using surface recognition software, as is known in the art, the sampled surface can be automatically matched with the corresponding surface in a pre-acquired image. Surface registration may be combined with point registration, as a follow-on step to increase accuracy.
Another technique for registering two different modalities is by a “path registration” technique. The three dimensional path of a natural or artificial conduit in an organ or region of a patient is obtained in a pre-operative scan using an imaging device. A position indicating element is then dragged through the pathway and its coordinates sampled in the patient space by a tracking device. It is then possible to register the shape of the path from the pre-operative image (image space) to the measured path of the sampled coordinates of the position indicating element (patient space) to get an automatic path registration. This method and apparatus has been disclosed in U.S. patent application Ser. No. 11/059,336 (U.S. Patent Publication No. 20050182319), entitled “Method and Apparatus for Registration, Verification, and Referencing of Internal Organs,” which is hereby incorporated by reference herein in its entirety.
Another type of registration is called “intrinsic registration,” in which a patient is essentially imaged in the same position as the procedure being performed on him or her. Examples of this type of registration are the so-called “fluoroscopic navigation” techniques.
A further type of registration process involves 2D/3D registration of a fluoroscopic image and a pre-operative scan. This minimally-invasive method merges the 3D pre-operative scan with the 2D image. For example, one way to automatically register a patient's heart with fluoroscopy and CT or other imaging modalities is to use nearby bones as anatomical landmarks, or to place objects or devices (such as skin markers) preoperatively so that they are visible in both the 2D and 3D images.
In an additional form of computer assisted surgery, images are not used and therefore no registration is required. In some situations, these “imageless” techniques may operate by determining the proximity of one tracked instrument relative to another or by using a tracked instrument to digitize a surface or a path.
Having performed the registration, it may be advantageous to track the motion of the registered anatomical object during the surgery, so that registration need be performed only once. This may be known as “dynamic referencing.” Tracking the motion of the registered anatomical object may also be advantageous to assist with registration. It would be advantageous to provide this tracking in the same device that is used for registration. Dynamic referencing may also be helpful for imageless techniques as it may compensate for gross patient motion which might be mistaken for other movement such as, for example, movement of one instrument toward another instrument, movement of an instrument toward a surface point, or other type of movement.
A process of “gating” may also be used to assist in imaging, registration, imageless computer assisted surgery, or for other purposes. During gating, a signal from a device capable of measuring physiological functions (such as heartbeat or respiration) may be used to trigger acquisition of an image from a scanner or acquisition of a sample from a position sensor or both (e.g., to selectively “gate” data acquisition according to the physiological function). Gating may be combined with traditional dynamic referencing to generate better anatomical dynamic referencing than simple dynamic referencing or gating alone.
Often, multiple additional devices may be attached to the skin of a patient to dynamically reference, register, gate, monitor, image, etc. This may cause problems, as these different devices compete for space in or on a patient.
Often an initial estimate of registration is useful for use in conjunction with other registration techniques that prove greater accuracy. For example, methods such as surface matching or path registration may make use of a mathematical technique known as “iterative closest points” (ICP). This technique is computationally intensive and converges much more rapidly and accurately when supplied with an initial estimate of registration. Such initial estimates may require a manual point-based registration which can be time consuming.
In some forms of computer assisted surgery, especially image-free techniques, hand-eye coordination is improved when a tracked surgical instrument moves on a computer display in the same direction as the surgeon expects it to move (e.g., on a coordinate system that is not rotated at an arbitrary angle relative to the patient). Often the intrinsic coordinate system of a dynamic reference is used as the coordinate system experienced by the surgeon. If a dynamic reference is placed in an arbitrary arrangement (as it usually is), the instrument “sense” (e.g., coordinate orientation) may be arbitrary, leading to difficulty manipulating the instrument (e.g., unexpected directional movement). Alternatively, arbitrary placement of the dynamic reference may require an additional step of rotating surgical images to compensate for such an arbitrary sense or to indicate to a computer the direction of the patient's head and feet so that the image sense is correctly displayed. Thus, a device that automatically corrects these problems would be desirable.
These and other problems exist.